1. Field of the Invention
The present invention relates to an innovative method of manufacturing forks for lift trucks and also to a plant for putting said method into practice.
2. State of the Prior Art
Manufacture of forks for lift trucks starting from segments of a steel bar of rectangular section is known in the art. At the beginning, the bar is tapered at an end thereof, then bending of the knee is carried out to obtain the traditional L-shaped configuration of the item; the fork is then heat-treated (with a hardening and tempering operation), then the fastening hooks are welded and finally the item is submitted to sandblasting, checking and painting operations. The bending process of the knee and heat treatment commonly used however do not allow forks to be obtained which correspond in size to the specifications. Therefore a process for straightening the fork after the heat treatment is often required.
In addition, after bending, grinding of the knee intrados and side faces is made necessary in order to eliminate burrs, surface unevennesses and defectivenesses, such as laps and more or less marked waving in the bent region of the fork, which, as known, is the most critical region in terms of fatigue strength.
Straightening and grinding make the fork manufacturing process much longer and particularly expensive, also taking into account that both operations can be hardly automated. Another problem connected with known processes resides in that straightening carried out under cold conditions induces residual tensile stresses that, added to the operating stresses, reduce the useful load of the fork. Straightening can also cause appearance of microcracks that can be hardly detected using traditional non destructive tests; the presence of these cracks drastically decreases reliability of the product as they constitute dangerous initiation points of fatigue failure during use.
In addition, the grinding operation can reduce the local resilient properties of the fork, as it involves work hardening in the ground region and locally modifies the material fibering obtained by hot plastic deformation.
It is a general aim of the present invention to obviate the above mentioned drawbacks by providing a method and a plant enabling manufacture of the fork in a quick, cheap and highly automated manner.
It is a further aim of the invention to provided a method and a plant enabling manufacture of a fork having satisfactory mechanical features (ultimate tensile stress, fatigue strength) and high reliability.